As the first Southern primary, South Carolina has been a proving ground for Republican presidential hopefuls in recent years.
Since Ronald Reagan in 1980, every Republican contender who won the primary has gone on to capture the party's nomination.
Romney's stumbles began even before his New Hampshire primary victory, when he told one audience that he had worried earlier in his career about the possibility of being laid off.
He gave a somewhat rambling, noncommittal response in a debate in Myrtle Beach last Monday when asked if he would release his tax returns before the primary. The following day, he told reporters that because most of his earnings come from investments, he paid about 15 percent of his income in taxes, roughly half the rate paid by millions of middle-class wage-earners. A day later, aides confirmed that some of his millions are invested in the Cayman Islands, although they said he did not use the offshore accounts as a tax haven.
Asked again at a debate in North Charleston on Thursday about releasing his taxes, his answer was anything but succinct and the audience appeared to boo.
Gingrich benefited from a shift in strategy that recalled his approach when he briefly soared to the top of the polls in Iowa. At mid-week he began airing a television commercial that dropped all references to Romney and his other rivals, and contended that he was the only Republican who could defeat Obama.
It featured several seconds from the first debate in which the audience cheered as he accused Obama of having put more Americans on food stamps than any other president.
Nor did Gingrich flinch when ex-wife Marianne said in an interview on ABC that he had been unfaithful for years before their divorce in 1999, and asked him for an open marriage.
Asked about the accusation in the opening moments of the second debate of the week, he unleashed an attack on ABC and debate host CNN and accused the "liberal news media" of trying to help Obama by attacking Republicans. His ex-wife's account, he said, was untrue.
Associated Press writers Shannon McCaffrey, Kasie Hunt and Beth Fouhy contributed to this report.